1. Field of the Invention
The present invention relates to a process for purifying a gas, in particular a plasmagenic rare gas, intended for example for the purification of krypton and xenon containing tetrafluoromethane and methane, and a purification apparatus for implementing such a process.
2. Description of the Related Art
Rare gases are generally extracted from air by distillation. After distillation, they contain impurities that have to be extracted before they are used.
Thus, in the case of krypton and xenon, the impurities mainly consist of tetrafluoromethane (CF.sub.4) and methane (CH.sub.4) at levels which may be as high as several hundreds of parts per million by volume (ppmv). The commonest applications of these rare gases require high-purity products and the presence of these foreign molecules is an unacceptable drawback for these applications.
For example, krypton is widely used for the partial-vacuum filling of filament lamps. In this application, the very high temperature to which the tungsten filament in these lamps is heated is sufficient to cause dissociation of the tetrafluoromethane into highly corrosive fluorinated radicals, which causes rapid degradation of the filament by etching of the metal.
Furthermore, methane is also dissociated under these temperature conditions and generates unstable gaseous precursors which form carbon-containing solid deposits of a brownish appearance on the internal surface of the bulb.
As is known and for technical reasons, the purification of rare gases cannot be accomplished by distillation.
The purification techniques known hitherto are essentially based on thermal phenomena.
Such techniques allow CH.sub.4 to decompose and/or oxidize easily, but they are unsuitable for eliminating CF.sub.4, which is an extremely stable and barely reactive molecule.
Moreover, since the CF.sub.4 molecule does not react significantly with any solid or liquid medium at room temperature, the chemical conversion of CF.sub.4 requires very high reaction temperatures to be reached and a considerable enthalpy to be transferred to the gaseous medium.
If a combustion technique is used, very high flow rates of combustible gas, generally hydrogen, are required, which raises treatment-cost problems, safety problems and plant-constraint problems because of the heat released. Moreover, such a technique is not suitable for reducing already low impurity concentrations to a negligible level.
Another known purification technique is based on thermochemical decomposition by reaction over a suitable metal, for example zirconium. This technique is relatively effective and does not require a very large supply of combustible gas. However, it has many disadvantages, in particular because of the fact that it is not possible to purify gases with a high flow rate and that its implementation requires an apparatus which takes up a lot of room.
Furthermore, this technique requires quite a high operating temperature, for example about 900 to 1000.degree. C., and the beds of metals used are very expensive and have a limited lifetime.
For the reasons explained above, the purification of rare gases currently constitutes the main cause of limited efficiency of lines for producing these gases.